The present invention relates to the growth of oxides on semiconductors and, in particular, relates to the differential growth of oxide layers over implanted areas of a silicon based substrate.
In the manufacture of semiconductor devices, thin layers of gate oxide are grown over active areas. It has long been known that slow oxide growth rates should be used to reproducibly grow quality thin oxide films of proper thicknesses over semiconductor dies. However, with the advent of new technologies there has been a need grow gate oxide layers of different thicknesses on different sections of a given semiconductor die. For example, in the manufacture of programmable devices some areas of a semiconductor die on a wafer may require gate oxide thicknesses of between 70 xc3x85 and 80 xc3x85. In other areas on the same semiconductor die, however, gate oxide thicknesses of 150 xc3x85 to 180 xc3x85 may be required. Historically, it has been difficult to control the proper oxide thicknesses within the separate regions of a given semiconductor die or wafer using conventional oxide growth methods.
In other applications, again in the manufacture of programmable devices, thin oxide layers are to be grown over areas of silicon that have been heavily doped. Oxidation rates for such areas are typically very much higher than for areas of undoped silicon. For example, areas of silicon that have been doped with phosphorus will exhibit an affinity for oxygen and so such areas of a semiconductor die will tend to oxidize much more quickly than undoped areas. Indeed, oxide growth rates over heavily doped areas of a semiconductor die have been observed that are five times faster than growth rates in undoped areas of the same semiconductor die.
Because of the need to precisely control the thickness of the gate oxide layer, there is a need to be able to control the oxidation rate over areas of heavily doped silicon. Ideally, one would like to keep the oxidation rate over such heavily doped areas approximately the same as the oxidation rate over undoped areas. This would provide for a very controlled oxide growth step during processing of the semiconductor die. At the very least, the oxidation rate for the heavily doped areas of silicon should be predictable and controllable.
The present invention provides, in one embodiment, a method of controlling the oxidation rate in a semiconductor die. Two different regions of the semiconductor die are implanted with different amounts of dopants/ions. The implantation may occur though a sacrificial oxide layer disposed over the semiconductor die. Following implantation in one or both regions, the semiconductor die may be annealed and the sacrificial oxide layer may be removed. An oxide layer may then be grown over the implanted region(s) of the semiconductor die.
For some embodiments, the semiconductor die may be implanted with arsenic and/or with phosphorus. Further, the anneal may be performed for approximately 30 to 120 minutes at a temperature between approximately 900xc2x0 C. and 950xc2x0 C.